Pickleable signals

[hunse]

Description:

This PR makes Signals pickle properly.

Motivation and context:

With the copy branch (#984), we can now start thinking about pickling whole Models. I tried this out, but the simulator steps were getting generated in random orders after reloading the Model. I tracked this down to the fact that when we pickle signals, specifically those that are views on other signals, the pickling process (I was using the dill package, since it is more flexible) didn't maintain views between the initial_value arrays of signals. Since we rely on this in the Signal.may_share_memory function, that was failing.

Interactions with other PRs:

This depends on #984.

How has this been tested?

Added a test which pickles a Model object and then unpickles it, ensuring that the simulation with both the original and re-created objects are the same.

Where should a reviewer start?

It's quite short.

How long should this take to review?

• Quick (less than 40 lines changed or changes are straightforward)

Types of changes:

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have read the CONTRIBUTING.rst document.
• [na] I have updated the documentation accordingly.
• I have included a changelog entry.
• I have added tests to cover my changes.
• All new and existing tests passed.

[hunse]

This is ready for review.